Starter motors to start automotive internal combustion (IC) engines require a heavy current for a short period of time. To connect this current to the motor, relays are used which must carry the heavy starting current required by the starter motor. Normally, the starter motor is operated only for very short periods of time so that, with respect to an equivalent continuous duty motor, the electrical components of the starter motor are under-designed to save costs; similarly, the contact elements of the starter relay, which do not carry current for a long time, should be designed, for economy reasons, for intermittent operation. Due to the difficult environmental operating conditions of automotive engines, and their starters, the relay must meet severe design requirements with respect to resistance to shock, vibration, and the like; they must be essentially sealed to prevent contamination by dampness, road salt, and the like; and they must be extremely reliable since many motor vehicles cannot be started unless the starter relay is operative. Conversely, as soon as energization current for the starter relay is disconnected, the circuit should be broken to prevent damage to the starter motor.
A starter relay of the type to which the present invention relates includes a housing which has a solenoid winding therein, in electromagnetic cooperation with a plunger-type armature core which can move axially with respect to the solenoid. The core is guided by a guide sleeve and carries a contact bridge element which, when the relay is energized, effects a cross connection between two contact terminals extending into a switching chamber, in which the contact bridge is operative. Springs are provided to reset the contact carrier bridge, or the core, or both, respectively, and maintain the contact bridge out of engagement with the fixed terminal contacts, unless the solenoid is energized.
In such an arrangement, a contact pressure spring can be provided to engage the longitudinally movable core and the contact carrying bridge to provide for reliable connection of the terminal contacts. It has been found that, if the contact bridge should stick to the fixed contact terminals, if the contact terminals should weld, or for other reasons the relay should "hang", it is very difficult to separate the contact bridge from one, or both, of the terminals so as to not only reliably connect, but also reliably interrupt starter motor current to the starter motor of the IC engine.